I was shooting a Relay for Life last night to 6 AM this morning at a local college and decided to use my Df. I had it set up for flash, AF-C single point, center point, focus priority. The Df struggled to lock focus in what I would consider in light, while low, not all that bad. I had to switch to AF-S, and the red AF-assist illuminator in order to lock focus. When I had a small break in the action I replaced the Df with my D800. With the same initial setup the D800 nailed the focus instantaneously 100% of the time.

At some point as light decreases, the D800 should come out ahead. I would be interested to know at what exposure (shutter, aperture, ISO) were you seeing the problem?

I took a picture with no flash, and it was f/4.0 at 1/80 at ISO 3200.

I just reread my DF manual on the (lack of an) AF assist light on the DF and the function of the selftimer (the red illiminator light) and think I now understand why your pictures with the DF were not in focus.

Apart from the low shutter speed, and the use of AF-S on moving subjects, given the minimum self timer speed of 2 (!!) seconds it would be amazing if you have anything which would resemble an in focus shot.

Basically you lock you focus on a subject which then moves away (however slow) from the spot you were aiming for during the millisecond between focus lock and shutter release, while the shutter actually opens at least two seconds (the minimum selftimer setting)after the shutter release was pushed.

I don't want to sound hasty, but unless the set up was chosen for artistic purposes, this really does not sound a fault of the DF, but rather a clear case of operator mistake.

Initial setup was using flash 100% of the time except for one test picture, AF-C, single point, priority focus. Switch to AF-S so I could use the red AF illuminator from the flash in order to get focus lock. I preferred AF-C, but had to switch to AF-S. I took one shot in available light (no flash) to get the reading. As soon as I had a break in the action, I went out to my car to get the D800, and finished the rest of the shoot with the D800.

Based on the info the OP now gives in detail, it's clear that he used flash all the time, and the red illuminator refers to the one on the flash. So in retrospect I was mistaken to think he was using his remote timer and the red light was that of the selftimer on the body.

But given the light circumstances I still think that even with AF-C sharp pictures would have been possible. Nikon gives its AF a sensitivity range of EV -1 to + 19. Even if this is judged as too optimistic, let's for discussion sake shave of 4 EV on the top and bottom, leaving a range of EV 3 to 15.

In real life shooting just before sunset corresponds to EV 12 to 14 and even after sunset to EV to 9 to 11 (source Wikipedia) while Fred Parker (http://fredparker.com/ultexp1.htm) gives sunset an EV 11 and even moonlight a higher EV, and calculates f4.0 at 1/80th at ISO 3200 to EV 5-6 ( I came on a higher EV when trying to calculate it on my Profisix), all values well within even the most pessimistic estimate of the AF sensitivity of the DF.

As already stated (also by others, not only me) for AF there are more factors which can influence its correct function:

(lack of) contrast of subject (AF needs some kind of contrast to be able to calculate the correct focus, and will go on tilt on e..g. a complete white or black, or due to lack of light low contrast subject)

use of a less sensitive AF field ( so in the case of the DF other then the center ones)

setting of AF (AF lock in AF-S mode with a moving subject)

flash synch ( first or second curtain when using a flash on a moving subject, especially with a moving subject and exposure close to availible light)

Auto ISO ( since the electronics will then select an ISO value corresponding to the availible light, which in combination with the flash and a slow shutter speed may very likely create a ghost image resembling unsharpness)

AF triggering via AF-On button or with the shutter release.

Having over the years shot more then I care to remember non-fashionweek catwalk under far from ideal lghting with AF-wise lowgrade bodies ranging from the F801to the F100, and DSLR's like the D1H and D70S, I have gained a well founded trust in the AF of Nikon in general, but always remain the one determining how it will function, rather then just dial in a setting and hope the system will do right.